<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        body {
            height: 100vh;
            width: 100%;
            margin: 0;
            padding: 0;
            background-color: black;
        }

        .box {
            box-sizing: border-box;
            margin: 100px auto;
            width: 200px;
            height: 200px;
            line-height: 200px;
            text-align: center;
            color: white;
            background-color: black;
            position: relative;
            font-size: 50px;
        }

        .box:before,
        .box:after {
            content: " ";
            position: absolute;
            left: -2px;
            right: -2px;
            top: -2px;
            bottom: -2px;
            background: linear-gradient(45deg, #4cd137, black, #3498db);
        }

        .box:before {
            z-index: -1;
        }

        .box:after {
            z-index: -2;
            filter: blur(30px);
        }
    </style>
</head>

<body>
    <div class="box">Cool</div>
</body>

</html>